The present invention relates to steam generation apparatus and, in particular, steam generation apparatus for secondary recovery of oil, a conversion unit for steam generation apparatus and methods for steam generation and conversion of steam generation apparatus.
Steam is often used in industrial processes. For example, steam can be used for heat exchange, as a power source for driving turbines, etc.
In the petroleum industry, for example, steam can be used for extraction processes and to enhance production. In one procedure, steam may be used for the recovery of bitumen or heavy oil from oil-bearing formations. A common process utilized for the in situ recovery of heavy oil or bitumen is to inject steam underground pursuant to which the viscosity of bitumen or heavy oil is decreased such that it flows and is capable of being pumped to the surface. For this, steam generation equipment commonly called steam injection boilers (“SIB”) are used to generate steam of the required/desired quality.
For in situ recovery of bitumen or heavy oil, prominent processes utilized are steam assisted gravity drainage (“SAGD”) and cyclic steam stimulation, with the SAGD process gaining in popularity due to it capabilities for enhanced recovery of bitumen or heavy oil. Generally, high quality steam of greater than 70% (i.e. 70% steam and 30% water) is generated by the boiler in specified volumes per hour depending on output capabilities of the boiler, as well as steam output requirements for the recovery and extraction process. Some processes generate/require 100's of thousands/lbs steam per hour. An 80% quality steam may commonly be used. Producing very high quality steam of greater than 80% quality typically results in escalating cost due to water treatment costs, potentially rendering a project uneconomical. Conversely, lower than 80% quality steam introduces inefficiencies to the process utilized for heavy oil or bitumen recovery and, hence, is also undesirable from a cost perspective.
Current SIB unit designs generally include horizontal cylindrical units including a combustion chamber with a burner at one end and steam generating coils therein, such as helical or serpentine steam generating coils, etc. Since almost all SIB units are fired with gaseous fuel (i.e. natural gas or liquid petroleum gas), these units are designed to suit the firing of this gaseous fuel.
Unfortunately, however, the gaseous fuel must often be piped significant distances to the location of steam generation, resulting in a significant cost to the producer due to the price of the gaseous fuel and the cost of the associated pipeline construction and maintenance. In fact, it has been stated that the economics associated with the in situ recovery of bitumen or heavy oil are primarily driven by the price of the gaseous fuel required to generate steam.
Thus, there is a desire in the industry to move to lower cost and/or more accessible fuels. The logical choice would be to fuel the steam generator using a small portion of the heavy oil or bitumen being produced at the site. However, conversion of SIB units from gas fuel to liquid fuel, such as heavy oil or bitumen, has been problematic for a number of reasons.
For example, since the flame resulting from firing natural gas is generally shorter than the flame resulting from firing liquid fuel, such as bitumen or heavy oil, the conversion of an existing SIB unit from gas firing to liquid fuel firing inevitably leads to lower firing rates as the combustion chamber of an existing SIB unit is only designed and sized to accommodate operating conditions incidental to gas firing. While lower firing rates of bitumen can be used and adjusted to mimic the gaseous fuel flame envelope size restrictions of the existing combustion chamber, these lower firing rates result in lower steam generating capacity, as well as lower quality steam (i.e. less than 80% quality). Combusting bitumen or heavy oil also requires the utilization of emission reduction/abatement technologies and equipment, as these liquids generally contain sulfur and other metallic components, resulting in undesirable by-products when combusted.
Even in new installations, the problems associated with liquid fuel firing has driven the industry to continue to use gaseous fuels. For example, a much larger combustion chamber is required for an oil-fired boiler to produce steam of the required quality and in the required amounts. This results in extra costs for equipment, transport and installation.